Nonaqueous electrolyte secondary battery separator

ABSTRACT

The present invention provides, as a separator having both a sufficient level of safety and sufficient strength, a nonaqueous electrolyte secondary battery separator including a polyolefin porous film, the nonaqueous electrolyte secondary battery separator being arranged such that in regard to a surface of the nonaqueous electrolyte secondary battery separator, a product obtained by multiplying (a) a difference between a surface roughness in a machine direction obtained by a contact measurement and a surface roughness in the machine direction obtained by a non-contact measurement by (b) a difference between a surface roughness in a transverse direction obtained by a contact measurement and a surface roughness in the transverse direction obtained by a non-contact measurement is not less than 0.0020 and not more than 0.0280.

This Nonprovisional application claims priority under 35 U.S.C. § 119 onPatent Application No. 2017-041094 filed in Japan on Mar. 3, 2017, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to (i) a separator for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery separator”), (ii) a laminated separatorfor a nonaqueous electrolyte secondary battery (hereinafter referred toas a “nonaqueous electrolyte secondary battery laminated separator”),(iii) a member for a nonaqueous electrolyte secondary battery(hereinafter referred to as a “nonaqueous electrolyte secondary batterymember”), and (iv) a nonaqueous electrolyte secondary battery.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries such as a lithium secondarybattery are currently in wide use as (i) batteries for devices such as apersonal computer, a mobile telephone, and a portable informationterminal or (ii) on-vehicle batteries.

As a separator for use in such a nonaqueous electrolyte secondarybattery, a porous film containing polyolefin as a main component ismainly used.

As a porous base material useful as a nonaqueous electrolyte secondarybattery separator, for example, Patent Literature 1 discloses apolyethylene microporous film whose flexion rate, porosity, and averagepore diameter are arranged to be in specific ranges, respectively, sothat a film thickness and a porosity, which are necessary for having asufficient strength, will be maintained and a high ion permeability willbe realized.

CITATION LIST Patent Literature

[Patent Literature 1] Japanese Patent Application Publication,Tokukaihei, No. 11-130900 (1999) (Publication Date: May 18, 1999)

SUMMARY OF INVENTION Technical Problem

However, a conventional nonaqueous electrolyte secondary batteryseparator as disclosed in Patent Literature was insufficient in that theconventional nonaqueous electrolyte secondary battery separator did notsimultaneously have both high strength and a low shutdown temperature(SD temperature).

Solution to Problem

The present invention encompasses aspects described in the following [1]to [5].

[1] A nonaqueous electrolyte secondary battery separator including apolyolefin porous film, wherein:

in regard to a surface of the nonaqueous electrolyte secondary batteryseparator, a product obtained by multiplying (a) a difference between asurface roughness in a machine direction obtained by a contactmeasurement and a surface roughness in the machine direction obtained bya non-contact measurement by (b) a difference between a surfaceroughness in a transverse direction obtained by a contact measurementand a surface roughness in the transverse direction obtained by anon-contact measurement is not less than 0.0020 and not more than0.0280.

[2] The nonaqueous electrolyte secondary battery separator as describedin [1], having a film thickness of not more than 19.5 μm.[3] A nonaqueous electrolyte secondary battery laminated separatorincluding a nonaqueous electrolyte secondary battery separator asdescribed in [1] or [2] and an insulating porous layer.[4] A nonaqueous electrolyte secondary battery member including: apositive electrode; a nonaqueous electrolyte secondary battery separatoras described in [1] or [2] or a nonaqueous electrolyte secondary batterylaminated separator as described in [3]; and a negative electrode, thepositive electrode, the nonaqueous electrolyte secondary batteryseparator or the nonaqueous electrolyte secondary battery laminatedseparator, and the negative electrode being disposed in this order.[5] A nonaqueous electrolyte secondary battery including: a nonaqueouselectrolyte secondary battery separator as described in [1] or [2] or anonaqueous electrolyte secondary battery laminated separator asdescribed in [3].

Advantageous Effects of Invention

Advantageously, a nonaqueous electrolyte secondary battery separator inaccordance with an embodiment of the present invention has both anexcellent piercing strength and an excellent shutdown temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram for explaining a method for calculating asurface roughness (Ra) of a nonaqueous electrolyte secondary batteryseparator in accordance with an embodiment of the present invention.

FIG. 2 is a schematic view (cross-sectional view) illustrating anexample of a configuration of a fibrous structure which constitutes aninternal structure of a nonaqueous electrolyte secondary batteryseparator.

FIG. 3 is a schematic view (cross-sectional view) illustrating anexample of a configuration of a fibrous structure which constitutes aninternal structure of a nonaqueous electrolyte secondary batteryseparator.

FIG. 4 is a schematic view (cross-sectional view) illustrating anexample of a configuration of a fibrous structure which constitutes aninternal structure of a nonaqueous electrolyte secondary batteryseparator.

DESCRIPTION OF EMBODIMENTS

The following description will discuss an embodiment of the presentinvention. The present invention is, however, not limited to theembodiment below. The present invention is not limited to thearrangements described below, but may be altered in various ways by askilled person within the scope of the claims. Any embodiment based on aproper combination of technical means disclosed in different embodimentsis also encompassed in the technical scope of the present invention.Note that numerical expressions such as “A to B” herein mean “not lessthan A and not more than B” unless otherwise stated.

Embodiment 1: Nonaqueous Electrolyte Secondary Battery Separator

A nonaqueous electrolyte secondary battery separator in accordance withEmbodiment 1 of the present invention is a nonaqueous electrolytesecondary battery separator including a polyolefin porous film, wherein:

in regard to a surface of the nonaqueous electrolyte secondary batteryseparator, a product obtained by multiplying (a) a difference between asurface roughness in a machine direction (hereinafter, referred to as“MD”) obtained by a contact measurement and a surface roughness in theMD obtained by a non-contact measurement by (b) a difference between asurface roughness in a transverse direction (hereinafter, referred to as“TD”) obtained by a contact measurement and a surface roughness in theTD obtained by a non-contact measurement is not less than 0.0020 and notmore than 0.0280.

Hereinafter, the surface roughness in the MD obtained by a contactmeasurement may be referred to as “Ra_(contact, MD)”, the surfaceroughness in the TD obtained by a contact measurement may be referred toas “Ra_(contact, TD)”, the surface roughness in the MD obtained by anon-contact measurement may be referred to as “Ra_(non-contact, MD)”,and the surface roughness in the TD obtained by a non-contactmeasurement may be referred to as “Ra_(non-contact, TD)”.

Note that the machine direction (MD) of a nonaqueous electrolytesecondary battery separator herein means a direction in which thenonaqueous electrolyte secondary battery separator is conveyed duringproduction. Meanwhile, the transverse direction (TD) of a nonaqueouselectrolyte secondary battery separator herein means a directionperpendicular to the MD of the nonaqueous electrolyte secondary batteryseparator.

The nonaqueous electrolyte secondary battery separator in accordancewith Embodiment 1 of the present invention includes a polyolefin porousfilm, and is preferably constituted by a polyolefin porous film. Note,here, that the “polyolefin porous film” is a porous film which containsa polyolefin-based resin as a main component. Note that the phrase“contains a polyolefin-based resin as a main component” means that aporous film contains a polyolefin-based resin at a proportion of notless than 50% by volume, preferably not less than 90% by volume, andmore preferably not less than 95% by volume, relative to the whole ofmaterials of which the porous film is made.

The polyolefin porous film can be the nonaqueous electrolyte secondarybattery separator in accordance with an embodiment of the presentinvention or a base material of a nonaqueous electrolyte secondarybattery laminated separator in accordance with an embodiment of thepresent invention, which will be described later. The polyolefin porousfilm has therein many pores, connected to one another, so that a gasand/or a liquid can pass through the polyolefin porous film from oneside to the other side.

The polyolefin-based resin more preferably contains a high molecularweight component having a weight-average molecular weight of 3×10⁵ to15×10⁶. In particular, the polyolefin-based resin more preferablycontains a high molecular weight component having a weight-averagemolecular weight of not less than 1,000,000 because the polyolefinporous film containing such a polyolefin-based resin and a nonaqueouselectrolyte secondary battery laminated separator including such apolyolefin porous film each have a higher strength.

Examples of the polyolefin-based resin which the polyolefin porous filmcontains as a main component include, but are not particularly limitedto, homopolymers (for example, polyethylene, polypropylene, andpolybutene) and copolymers (for example, ethylene-propylene copolymer)both of which are thermoplastic resins and are each produced through(co)polymerization of a monomer(s) such as ethylene, propylene,1-butene, 4-methyl-1-pentene, and/or 1-hexene. Among these, polyethyleneis preferable as it is capable of preventing (shutting down) a flow ofan excessively large electric current at a lower temperature.

Ra_(contact, MD) and Ra_(contact, TD) are each an average value ofvalues which are obtained by carrying out a contact measurement twotimes. Meanwhile, Ra_(non-contact, MD) and Ra_(non-contact, TD) are eachan average value of values which are obtained by carrying out anon-contact measurement five times.

The “surface roughness (Ra)” of the nonaqueous electrolyte secondarybattery separator in accordance with an embodiment of the presentinvention herein indicates a value obtained as follows, from a curve(roughness curve) representing an uneven form of a surface of thenonaqueous electrolyte secondary battery separator. That is, asillustrated in FIG. 1, the roughness curve is folded at the center linesuch that a portion of the roughness curve above a center line is foldeddown. Then, an area of a region thus formed is divided by a length. Aquotient thus obtained is expressed as a value in the unit of micrometer(μm). This value is the surface roughness.

The contact measurement is a method of measuring a surface roughness onthe basis of a degree of unevenness (a depth of a depression and aheight of a protrusion) of an uneven surface of the nonaqueouselectrolyte secondary battery separator. This degree of unevenness ismeasured on the basis of a distance by which a probe for measurementmoves up and down in accordance with the uneven surface of thenonaqueous electrolyte secondary battery separator when the prove iscaused to sweep along the uneven surface in contact with the unevensurface. Note that the contact measurement can be measured by acommercially available contact-type surface roughness measurementapparatus. An example of the contact-type surface roughness measurementapparatus includes “HANDYSURF (E-35A)” manufactured by Tokyo SeimitsuCo., Ltd. which was used in Examples described later. Further, thesurface roughness can be measured, for example, by the method describedin Examples.

Note that in the contact measurement, the probe for measurement comes incontact with the nonaqueous electrolyte secondary battery separator.Therefore, when the probe sweeps on the nonaqueous electrolyte secondarybattery separator, pressure is applied to a surface of the nonaqueouselectrolyte secondary battery separator. Then, the nonaqueouselectrolyte secondary battery separator deforms due to the pressure.This results in a difference between an actual degree of unevenness ofthe surface of the nonaqueous electrolyte secondary battery separatorand a measured degree of unevenness of that surface.

A degree of such deformation of the nonaqueous electrolyte secondarybattery separator depends on flexibility of the nonaqueous electrolytesecondary battery separator. The nonaqueous electrolyte secondarybattery separator here includes a network structure constituted by afibrous structure constituting a resin component. Therefore, in a casewhere the fibrous structure is more flexible, the nonaqueous electrolytesecondary battery separator deforms more. This results in a largerdifference between an actual degree of unevenness of the surface of thenonaqueous electrolyte secondary battery separator and a measured degreeof unevenness of that surface.

On the other hand, the non-contact measurement is a method of measuringa surface roughness on the basis of a degree of unevenness of a surfaceof a nonaqueous electrolyte secondary battery separator, which degree ofunevenness is measured without causing a measurement apparatus to comein contact with the nonaqueous electrolyte secondary battery separator.There are various methods known as a method of such a non-contactmeasurement. Examples of the non-contact measurement include a method ofmeasuring a surface roughness on the basis of a degree of unevenness ofa surface of a nonaqueous electrolyte secondary battery separator, whichdegree of unevenness is measured on the basis of interference of lightbetween irradiation light and reflected light, which interference oflight is produced by, for example, (i) irradiating the surface of thenonaqueous electrolyte secondary battery separator with white light(irradiation light) and causing reflection of the irradiation light bythe surface of the nonaqueous electrolyte secondary battery separator.More specifically, for example, the non-contact measurement can becarried out by a method described in Examples (described later) by usinga non-contact-type roughness measurement apparatus (“VertScan[registered trademark] 2.0 R5500GML” manufactured by Ryoka Systems Inc.)described in the Examples.

In the non-contact measurement, the nonaqueous electrolyte secondarybattery separator is never deformed by measurement. Therefore, ameasured degree of unevenness of a surface of a nonaqueous electrolytesecondary battery separator becomes equal to an actual degree ofunevenness of the surface of the nonaqueous electrolyte secondarybattery separator.

Therefore, a larger “product of Ra differences” of the nonaqueouselectrolyte secondary battery separator in accordance with an embodimentof the present invention means that the nonaqueous electrolyte secondarybattery separator and the fibrous structure contained in the nonaqueouselectrolyte secondary battery separator are more flexible.

The flexibility of the nonaqueous electrolyte secondary batteryseparator, that is, the flexibility of the fibrous structure is largelyrelated to a degree of crystallinity of this fibrous structure.

Specifically, as illustrated in FIGS. 2 to 4, the structure of thefibrous structure includes a surface layer portion 1 and an interiorportion 2. The surface layer portion 1 is mainly made of a crystallineportion whereas the interior portion 2 is mainly made of anon-crystalline portion. Further, the crystalline portion is harder,stronger, and more difficult to melt, as compared to the non-crystallineportion.

Accordingly, in a case where a proportion of the crystalline portion tothe whole of the fibrous structure, that is, a proportion of the surfacelayer portion 1 to the whole of the fibrous structure is higher asillustrated in FIG. 3, the fibrous structure becomes harder and the“product of Ra differences” becomes smaller. In contrast, in a casewhere a proportion of the non-crystalline portion to the whole of thefibrous structure, that is, a proportion of the interior portion 2 tothe whole of the fibrous structure is higher as illustrated in FIG. 4,the fibrous structure is more flexible and the “product of Radifferences” is larger.

Therefore, when the “product of Ra differences” of the nonaqueouselectrolyte secondary battery separator in accordance with an embodimentof the present invention is not less than 0.0020, the proportion of thenon-crystalline portion contained in the fibrous structure is higher.This allows the nonaqueous electrolyte secondary battery separatorincluding the fibrous structure to have a sufficiently low SDtemperature and an enhanced level of safety. In view of the above, the“product of Ra differences” is preferably not less than 0.0025 and morepreferably not less than 0.0030.

Meanwhile, the strength (e.g., piercing strength) of the nonaqueouselectrolyte secondary battery separator depends on the proportion of thecrystalline portion contained in the fibrous structure. Therefore, whenthe “product of Ra differences” of the nonaqueous electrolyte secondarybattery separator in accordance with an embodiment of the presentinvention is not more than 0.0280, the proportion of the crystallineportion contained in the fibrous structure becomes higher. This allowsthe nonaqueous electrolyte secondary battery separator to have asufficiently increased strength. In view of the above, the “product ofRa differences” is preferably not more than 0.0250 and more preferablynot more than 0.0200.

Therefore, when the “product of Ra differences” of the nonaqueouselectrolyte secondary battery separator in accordance with an embodimentof the present invention is not less than 0.0020 and not more than0.0280, the surface layer portion 1 and the interior portion 2 (thecrystalline portion and the non-crystalline portion) constituting thefibrous structure are well-balanced as illustrated in FIG. 2, so thatboth the strength and the level of safety of the nonaqueous electrolytesecondary battery separator can be kept favorable in a well-balancedmanner.

The nonaqueous electrolyte secondary battery separator in accordancewith an embodiment of the present invention preferably has a smallerdifference between (a) a difference between a surface roughnessRa_(contact, MD) in the MD obtained by a contact measurement and asurface roughness Ra_(non-contact, MD) in the MD obtained by anon-contact measurement (|Ra_(contact, MD)−Ra_(non-contact, MD)|) and(b) a difference between a surface roughness Ra_(contact, TD) in the TDobtained by a contact measurement and a surface roughnessRa_(non-contact, TD) in the TD obtained by a non-contact measurement(|Ra_(contact, TD)−Ra_(non-contact, TD)|) This is because, when thedifference is smaller, the nonaqueous electrolyte secondary batteryseparator is more isotropic. Specifically, it is preferable that both of|Ra_(contact, MD)−Ra_(non-contact, MD)| and|Ra_(contact, TD)−Ra_(non-contact, TD)| be in a range of not less than0.04 and not more than 0.30.

The thickness of the polyolefin porous film is not particularly limited,but is preferably 4 μm to 40 μm, and more preferably 5 μm to 20 μm.

The thickness of the polyolefin porous film is preferably not less than4 μm since an internal short circuit can be sufficiently prevented withsuch a thickness.

On the other hand, the thickness of the polyolefin porous film ispreferably not more than 40 μm since an increase in size of a nonaqueouselectrolyte secondary battery can be prevented with such a thickness.

Further, the thickness of the nonaqueous electrolyte secondary batteryseparator in accordance with an embodiment of the present invention isparticularly preferably not more than 19.5 μm, in view of reduction insize and increase in capacity of the nonaqueous electrolyte secondarybattery separator.

The polyolefin porous film typically has a weight per unit area ofpreferably 4 g/m² to 20 g/m², and more preferably 5 g/m² to 12 g/m², soas to allow a nonaqueous electrolyte secondary battery to have a higherweight energy density and a higher volume energy density.

The polyolefin porous film has an air permeability of preferably 30sec/100 mL to 500 sec/100 mL, and more preferably 50 sec/100 mL to 300sec/100 mL, in terms of Gurley values, since a sufficient ionpermeability is exhibited with such an air permeability.

The polyolefin porous film has a porosity of preferably 20% by volume to80% by volume, and more preferably 30% by volume to 75% by volume, so asto (i) retain a larger amount of electrolyte and (ii) obtain thefunction of reliably preventing (shutting down) a flow of an excessivelylarge electric current at a lower temperature.

The polyolefin porous film has a pore diameter of preferably not morethan 0.3 μm and more preferably not more than 0.14 μm, in view ofsufficient ion permeability and of preventing particles which constitutean electrode from entering the polyolefin porous film.

The nonaqueous electrolyte secondary battery separator in accordancewith an embodiment of the present invention may include a porous layeras needed, in addition to the polyolefin porous film. Examples of theporous layer encompass an insulating porous layer constituting thenonaqueous electrolyte laminated separator (described later) andpublicly known porous layers such as a heat-resistant layer, an adhesivelayer, and a protective layer.

[Method for Producing Polyolefin Porous Film]

Examples of a method for producing the polyolefin porous film include,but are not particularly limited to, a method in which apolyolefin-based resin and an additive are mixed and melt-kneaded andthen extruded through a T-die to obtain a polyolefin resin composition,and the polyolefin resin composition is subjected to stretching,cleaning and drying.

Specifically, the method can be a method including the following stepsof:

(A) melt-kneading a polyolefin-based resin and an additive in a kneaderto obtain a molten mixture;(B) extruding, through a T-die, the molten mixture having been obtainedin the step (A), and then shaping the molten mixture into a sheet whilecooling the molten mixture, so that a sheet-shaped polyolefin resincomposition is obtained;(C) stretching the sheet-shaped polyolefin resin composition having beenobtained in the step (B);(D) cleaning, with use of a cleaning liquid, the polyolefin resincomposition having been stretched in the step (C); and(E) drying and/or heat fixing the polyolefin resin composition havingbeen cleaned in the step (D), so that a polyolefin porous film isobtained.

In the step (A), the polyolefin-based resin is used in an amount ofpreferably 6% by weight to 45% by weight, and more preferably 9% byweight to 36% by weight, with respect to 100% by weight of thepolyolefin resin composition to be obtained.

Examples of the additive in the step (A) include: water-solubleinorganic compounds such as calcium carbonate; phthalate esters such asdioctyl phthalate; unsaturated higher alcohol such as oleyl alcohol;saturated higher alcohol such as stearyl alcohol; low molecular weightpolyolefin-based resin such as paraffin wax; petroleum resin; and liquidparaffin.

Examples of the petroleum resin include: (i) an aliphatic hydrocarbonresin obtained through polymerization of a C5 petroleum fraction such asisoprene, pentene, and pentadiene as a main material; (ii) an aromatichydrocarbon resin obtained through polymerization of a C9 petroleumfraction such as indene, vinyltoluene, and methyl styrene as a mainmaterial; (iii) copolymer resins of the aliphatic hydrocarbon resin andthe aromatic hydrocarbon resin; (iv) alicyclic saturated hydrocarbonresins obtained through hydrogenation of the resins (i) to (iii); and(v) varying mixtures of the resins (i) to (iv). These additives may beused alone or may be used in combination. Among these additives, acombination of (i) liquid paraffin, which serves as a pore formingagent, and (ii) a petroleum resin is preferably used.

In the step (A), an additive such as liquid paraffin may be added afterthe polyolefin-based resin is heated and kneaded. In this case, thetemperature inside the kneader at the time when liquid paraffin is addedis preferably not lower than 140° C. and not higher than 200° C. andmore preferably not lower than 172° C. and not higher than 190° C. Notethat the temperature inside the kneader is an average temperature oftemperatures in three segment barrels, one of which is providedimmediately before a section where liquid paraffin is added, another oneof which is provided at a section where the liquid paraffin is put in,and the other one of which is provided immediately after the sectionwhere the liquid paraffin is put in.

In cooling in the step (B), it is preferable to use, for example, amethod in which the molten mixture is put in contact with a coolingroller.

The cooling roller is preferably at a temperature of 5° C. to 60° C. Thecooling roller is preferably at a circumferential velocity of 0.1 m/minto 30 m/min, and more preferably 0.5 m/min to 10 m/min.

In the step (C), the sheet-shaped polyolefin resin composition isstretched at a stretch ratio of preferably not less than 3.0 times andnot more than 7.0 times, and more preferably not less than 4.5 times andnot more than 6.5 times.

Note that the polyolefin resin composition extruded through the T-diecan be stretched by sequential biaxial stretching in which thepolyolefin resin composition is first stretched in the MD and thenstretched in the TD or by another alternative stretching method. Asanother stretching method, there is, for example, simultaneous biaxialstretching in which the polyolefin resin composition is simultaneouslystretched in the MD and the TD.

Examples of the cleaning liquid used in the step (D) include, but arenot particularly limited to, hydrocarbon compounds such as heptane anddecane; and halogenated hydrocarbon compounds such as methylenechloride.

In the step (E), the polyolefin resin composition having been cleaned isdried and/or subjected to heat treatment at a specific temperature forheat fixing. A drying temperature during the drying is preferably roomtemperature. The heat fixing is performed at a temperature of preferablynot less than 110° C. and not more than 140° C., and more preferably notless than 115° C. and not more than 135° C. Further, the heat fixing isperformed for a period of preferably not less than 0.5 minutes and notmore than 60 minutes, and more preferably not less than 1 minute and notmore than 30 minutes.

In the method for producing a polyolefin porous film contained in thenonaqueous electrolyte secondary battery separator in accordance with anembodiment of the present invention, it is possible to control aninternal structure of a porous film to be obtained, particularly, adegree of crystallinity of a constituent resin component in a preferablerange by (i) adding a petroleum resin as the additive in the step (A)and further (ii) controlling the temperature and the circumferentialvelocity of the cooling roller in the above-described ranges,respectively, in the step (B). As a result, it is possible to suitablycontrol a difference between (i) a surface roughness of the nonaqueouselectrolyte secondary battery separator measured by a contactmeasurement method and (ii) a surface roughness of the nonaqueouselectrolyte secondary battery separator measured by a non-contactmeasurement method.

Embodiment 2: Nonaqueous Electrolyte Secondary Battery LaminatedSeparator

A nonaqueous electrolyte secondary battery laminated separator inaccordance with Embodiment 2 of the present invention includes (i) anonaqueous electrolyte secondary battery separator in accordance withEmbodiment 1 of the present invention and (ii) an insulating porouslayer. Accordingly, the nonaqueous electrolyte secondary batterylaminated separator in accordance with Embodiment 2 of the presentinvention includes a polyolefin porous film constituting theabove-described nonaqueous electrolyte secondary battery separator inaccordance with Embodiment 1 of the present invention.

[Insulating Porous Layer]

The insulating porous layer constituting the nonaqueous electrolytesecondary battery laminated separator in accordance with an embodimentof the present invention is typically a resin layer containing a resin.This insulating porous layer is preferably a heat-resistant layer or anadhesive layer. The insulating porous layer (hereinafter, also referredto as simply “porous layer”) preferably contains a resin that isinsoluble in a nonaqueous electrolyte of a battery and that iselectrochemically stable when the battery is in normal use.

The porous layer is provided on one surface or both surfaces of thenonaqueous electrolyte secondary battery separator as needed. In a casewhere the porous layer is provided on one surface of the polyolefinporous film, the porous layer is preferably provided on that surface ofthe polyolefin porous film which surface faces a positive electrode of anonaqueous electrolyte secondary battery to be produced, more preferablyon that surface of the polyolefin porous film which surface comes intocontact with the positive electrode.

Examples of the resin constituting the porous layer encompasspolyolefins; (meth)acrylate-based resins; fluorine-containing resins;polyamide-based resins; polyester-based resins; polyimide-based resins;rubbers; resins with a melting point or glass transition temperature ofnot lower than 180° C.; and water-soluble polymers.

Among the above resins, polyolefins, acrylate-based resins,fluorine-containing resins, polyamide-based resins, polyester-basedresins and water-soluble polymers are preferable. As the polyamideresins, wholly aromatic polyamides (aramid resins) are preferable. Asthe polyester-based resins, polyarylates and liquid crystal polyestersare preferable.

The porous layer may contain fine particles. The term “fine particles”herein means organic fine particles or inorganic fine particlesgenerally referred to as a filler. Therefore, in a case where the porouslayer contains fine particles, the above resin contained in the porouslayer has a function as a binder resin for binding (i) fine particlestogether and (ii) fine particles and the porous film. The fine particlesare preferably electrically insulating fine particles.

Examples of the organic fine particles contained in the porous layerencompass resin fine particles.

Specific examples of the inorganic fine particles contained in theporous layer encompass fillers made of inorganic matters such as calciumcarbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth,magnesium carbonate, barium carbonate, calcium sulfate, magnesiumsulfate, barium sulfate, aluminum hydroxide, boehmite, magnesiumhydroxide, calcium oxide, magnesium oxide, titanium oxide, titaniumnitride, alumina (aluminum oxide), aluminum nitride, mica, zeolite, andglass. These inorganic fine particles are electrically insulating fineparticles. The porous layer may contain only one kind of the fineparticles or two or more kinds of the fine particles in combination.

Among the above fine particles, fine particles made of an inorganicmatter is suitable. Fine particles made of an inorganic oxide such assilica, calcium oxide, magnesium oxide, titanium oxide, alumina, mica,zeolite, aluminum hydroxide, or boehmite are preferable. Further, fineparticles made of at least one kind selected from the group consistingof silica, magnesium oxide, titanium oxide, aluminum hydroxide,boehmite, and alumina are more preferable. Fine particles made ofalumina are particularly preferable.

A fine particle content of the porous layer is preferably 1% by volumeto 99% by volume, and more preferably 5% by volume to 95% by volume withrespect to 100% by volume of the porous layer. In a case where the fineparticle content falls within the above range, it is less likely for avoid, which is formed when fine particles come into contact with eachother, to be blocked by a resin or the like. This makes it possible toachieve sufficient ion permeability and a proper weight per unit area ofthe porous film.

The porous layer may include a combination of two or more kinds of fineparticles which differ from each other in particle or specific surfacearea.

A thickness of the porous layer is preferably 0.5 μm to 15 μm (persurface of the nonaqueous electrolyte secondary battery laminatedseparator), and more preferably 2 μm to 10 μm (per surface of thenonaqueous electrolyte secondary battery laminated separator).

If the thickness of the porous layer is less than 1 μm, it may not bepossible to sufficiently prevent an internal short circuit caused bybreakage or the like of a battery. In addition, an amount of electrolytesolution to be retained by the porous layer may decrease. In contrast,if a total thickness of porous layers on both surfaces of the nonaqueouselectrolyte secondary battery separator is above 30 μm, then a ratecharacteristic or a cycle characteristic may deteriorate.

The weight per unit area of the porous layer (per surface of thenonaqueous electrolyte secondary battery laminated separator) ispreferably 1 g/m² to 20 g/m², and more preferably 4 g/m² to 10 g/m².

A volume per square meter of a porous layer constituent componentcontained in the porous layer (per surface of the nonaqueous electrolytesecondary battery laminated separator) is preferably 0.5 cm³ to 20 cm³,more preferably 1 cm³ to 10 cm³, and still more preferably 2 cm³ to 7cm³.

For the purpose of obtaining sufficient ion permeability, a porosity ofthe porous layer is preferably 20% by volume to 90% by volume, and morepreferably 30% by volume to 80% by volume. In order for a nonaqueouselectrolyte secondary battery laminated separator to obtain sufficiention permeability, a pore diameter of each of pores of the porous layeris preferably not more than 3 μm, and more preferably not more than 1μm.

[Laminated Body]

A laminated body which is the nonaqueous electrolyte secondary batterylaminated separator in accordance with Embodiment 2 of the presentinvention includes a nonaqueous electrolyte secondary battery separatorin accordance with an embodiment of the present invention and aninsulating porous layer. The laminated body is preferably arranged suchthat the above-described insulating porous layer is provided on onesurface or both surfaces of the nonaqueous electrolyte secondary batteryseparator in accordance with an embodiment of the present invention.

The laminated body in accordance with an embodiment of the presentinvention has a thickness of preferably 5.5 μm to 45 μm, and morepreferably 6 μm to 25 μm.

The laminated body in accordance with an embodiment of the presentinvention has an air permeability of preferably 30 sec/100 mL to 1000sec/100 mL, and more preferably 50 sec/100 mL to 800 sec/100 mL, interms of Gurley values.

The laminated body in accordance with an embodiment of the presentinvention may include, in addition to the polyolefin porous film and theinsulating porous layer which are described above, a publicly knownporous film(s) (porous layer(s)) such as a heat-resistant layer, anadhesive layer, and a protective layer according to need as long as sucha porous film does not prevent an object of an embodiment of the presentinvention from being attained.

The laminated body in accordance with an embodiment of the presentinvention includes, as a base material, a nonaqueous electrolytesecondary battery separator having the above-described “product of Radifferences” in a specific range. Therefore, while the strength of thelaminated body is maintained, the nonaqueous electrolyte secondarybattery including the laminated body as a nonaqueous electrolytesecondary battery laminated separator can have an enhanced level ofsafety.

[Method for Producing Porous Layer and Method for Producing LaminatedBody]

The insulating porous layer in accordance with an embodiment of thepresent invention and the laminated body in accordance with anembodiment of the present invention can be each produced by, forexample, applying a coating solution (described later) to a surface ofthe polyolefin porous film of the nonaqueous electrolyte secondarybattery separator in accordance with an embodiment of the presentinvention and then drying the coating solution so as to deposit theinsulating porous layer.

Prior to applying the coating solution to a surface of the polyolefinporous film of the nonaqueous electrolyte secondary battery separator inaccordance with an embodiment of the present invention, the surface towhich the coating solution is to be applied can be subjected to ahydrophilization treatment as needed.

The coating solution for use in a method for producing the porous layerin accordance with an embodiment of the present invention and a methodfor producing the laminated body in accordance with an embodiment of thepresent invention can be prepared typically by (i) dissolving, in asolvent, a resin that may be contained in the porous layer describedabove and (ii) dispersing, in the solvent, fine particles that may becontained in the porous layer described above. The solvent in which theresin is to be dissolved here also serves as a dispersion medium inwhich the fine particles are to be dispersed. Depending on the solvent,the resin may be an emulsion.

The solvent (dispersion medium) is not limited to any particular one,provided that (i) the solvent does not have an adverse effect on thepolyolefin porous film, (ii) the solvent allows the resin to beuniformly and stably dissolved in the solvent, and (iii) the solventallows the fine particles to be uniformly and stably dispersed in thesolvent. Specific examples of the solvent (dispersion medium) encompasswater and organic solvents. Only one of these solvents can be used, ortwo or more of these solvents can be used in combination.

The coating solution may be formed by any method, provided that thecoating solution can meet conditions, such as a resin solid content(resin concentration) and a fine particle amount, which are necessaryfor obtaining a desired porous layer. Specific examples of the method offorming the coating solution encompass a mechanical stirring method, anultrasonic dispersion method, a high-pressure dispersion method, and amedia dispersion method. Further, the coating solution may contain, as acomponent(s) other than the resin and the fine particles, an additive(s)such as a disperser, a plasticizer, a surfactant, and/or a pH adjustor,provided that the additive does not prevent the object of an embodimentof the present invention from being attained. Note that the additive maybe contained in an amount that does not prevent the object of anembodiment of the present invention from being attained.

A method of applying the coating solution to the porous film, that is, amethod of forming a porous layer on a surface of the polyolefin porousfilm is not limited to any particular one. The porous layer can beformed by, for example, (i) a method including the steps of applying thecoating solution directly to a surface of the polyolefin porous film andthen removing the solvent (dispersion medium), (ii) a method includingthe steps of applying the coating solution to an appropriate support,removing the solvent (dispersion medium) for formation of a porouslayer, then pressure-bonding the porous layer to the polyolefin porousfilm, and subsequently peeling the support off, and (iii) a methodincluding the steps of applying the coating solution to a surface of anappropriate support, then pressure-bonding the polyolefin porous film tothat surface, then peeling the support off, and subsequently removingthe solvent (dispersion medium).

The coating solution can be applied by a conventionally publicly knownmethod. Specific examples of such a method include a gravure coatermethod, a dip coater method, a bar coater method, and a die coatermethod.

The solvent (dispersion medium) is generally removed by a drying method.The solvent (dispersion medium) contained in the coating solution may bereplaced with another solvent before a drying operation.

Embodiment 3: Nonaqueous Electrolyte Secondary Battery Member, andEmbodiment 4: Nonaqueous Electrolyte Secondary Battery

A nonaqueous electrolyte secondary battery member in accordance withEmbodiment 3 of the present invention is obtained by including apositive electrode, a nonaqueous electrolyte secondary battery separatorin accordance with Embodiment 1 of the present invention or a nonaqueouselectrolyte secondary battery laminated separator in accordance withEmbodiment 2 of the present invention, and a negative electrode, thepositive electrode, the nonaqueous electrolyte secondary batteryseparator or the nonaqueous electrolyte secondary battery laminatedseparator, and the negative electrode being disposed in this order.

A nonaqueous electrolyte secondary battery in accordance with Embodiment4 of the present invention includes the nonaqueous electrolyte secondarybattery separator in accordance with Embodiment 1 of the presentinvention or the nonaqueous electrolyte secondary battery laminatedseparator in accordance with Embodiment 2 of the present invention.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can be, for example, a nonaqueoussecondary battery that achieves an electromotive force through dopingwith and dedoping of lithium, and can include a nonaqueous electrolytesecondary battery member including a positive electrode, a nonaqueouselectrolyte secondary battery separator in accordance with an embodimentof the present invention, and a negative electrode, the positiveelectrode, the nonaqueous electrolyte secondary battery separator, andthe negative electrode being disposed in this order. Alternatively, thenonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can be, for example, a nonaqueoussecondary battery that achieves an electromotive force through dopingwith and dedoping of lithium, and can be a lithium-ion secondary batterythat includes a nonaqueous electrolyte secondary battery memberincluding a positive electrode, a porous layer, a nonaqueous electrolytesecondary battery separator in accordance with an embodiment of thepresent invention, and a negative electrode which are disposed in thisorder, that is, a lithium-ion secondary battery that includes anonaqueous electrolyte secondary battery member including a positiveelectrode, a nonaqueous electrolyte secondary battery laminatedseparator in accordance with an embodiment of the present invention, anda negative electrode which are disposed in this order. Note thatconstituent elements, other than the nonaqueous electrolyte secondarybattery separator, of the nonaqueous electrolyte secondary battery arenot limited to those described below.

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention is typically arranged so that abattery element is enclosed in an exterior member, the battery elementincluding (i) a structure in which the negative electrode and thepositive electrode faces each other via the nonaqueous electrolytesecondary battery separator in accordance with an embodiment of thepresent invention or the nonaqueous electrolyte secondary batterylaminated separator in accordance with an embodiment of the presentinvention and (ii) an electrolyte with which the structure isimpregnated. The nonaqueous electrolyte secondary battery is preferablya secondary battery including a nonaqueous electrolyte, and isparticularly preferably a lithium-ion secondary battery. Note that thedoping means occlusion, support, adsorption, or insertion, and means aphenomenon in which lithium ions enter an active material of anelectrode (e.g., a positive electrode).

Since the nonaqueous electrolyte secondary battery member in accordancewith an embodiment of the present invention includes the nonaqueouselectrolyte secondary battery separator in accordance with an embodimentof the present invention or the nonaqueous electrolyte secondary batterylaminated separator in accordance with an embodiment of the presentinvention, the strength and the level of safety of a nonaqueouselectrolyte secondary battery can be improved in a well-balanced mannerin a case where the nonaqueous electrolyte secondary battery separatormember is included in the nonaqueous electrolyte secondary battery.Since the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes the nonaqueous electrolytesecondary battery separator in accordance with an embodiment of thepresent invention which has the above-described “product of Radifferences” adjusted in a specific range, the nonaqueous electrolytesecondary battery advantageously has both excellent strength and anexcellent level of safety.

<Positive Electrode>

A positive electrode included in the nonaqueous electrolyte secondarybattery member in accordance with an embodiment of the present inventionor in the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention is not limited to any particularone, provided that the positive electrode is one that is generally usedas a positive electrode of a nonaqueous electrolyte secondary battery.Examples of the positive electrode encompass a positive electrode sheethaving a structure in which an active material layer containing apositive electrode active material and a binder resin is formed on acurrent collector. The active material layer may further contain anelectrically conductive agent and/or a binding agent.

The positive electrode active material is, for example, a materialcapable of being doped with and dedoped of lithium ions. Specificexamples of such a material encompass a lithium complex oxide containingat least one transition metal such as V, Mn, Fe, Co, or Ni.

Examples of the electrically conductive agent include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and a fired product of anorganic polymer compound. It is possible to use only one kind of theabove electrically conductive agents or two or more kinds of the aboveelectrically conductive agents in combination.

Examples of the binding agent encompass (i) fluorine-based resins suchas polyvinylidene fluoride, (ii) acrylic resin, and (iii) styrenebutadiene rubber. Note that the binding agent serves also as athickener.

Examples of the positive electrode current collector encompass electricconductors such as Al, Ni, and stainless steel. Among these, Al ispreferable because Al is easily processed into a thin film and isinexpensive.

Examples of a method for producing the positive electrode sheetencompass: a method in which a positive electrode active material, anelectrically conductive agent, and a binding agent are pressure-moldedon a positive electrode current collector; and a method in which (i) apositive electrode active agent, an electrically conductive agent, and abinding agent are formed into a paste with the use of a suitable organicsolvent, (ii) then, a positive electrode current collector is coatedwith the paste, and (iii) subsequently, the paste is dried and thenpressured so that the paste is firmly fixed to the positive electrodecurrent collector.

<Negative Electrode>

A negative electrode included in the nonaqueous electrolyte secondarybattery member in accordance with an embodiment of the present inventionor in the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention is not limited to any particularone, provided that the negative electrode is one that is generally usedas a negative electrode of a nonaqueous electrolyte secondary battery.Examples of the negative electrode encompass a negative electrode sheethaving a structure in which an active material layer containing anegative electrode active material and a binder resin is formed on acurrent collector. The active material layer may further contain anelectrically conductive agent.

Examples of the negative electrode active material encompass (i) amaterial capable of being doped with and dedoped of lithium ions, (ii)lithium metal, and (iii) lithium alloy. Examples of such a materialencompass carbonaceous materials such as natural graphite, artificialgraphite, cokes, carbon black, and pyrolytic carbon.

Examples of the negative electrode current collector encompass Cu, Ni,and stainless steel. Among these, Cu is more preferable because Cu isnot easily alloyed with lithium especially in a lithium-ion secondarybattery and is easily processed into a thin film.

Examples of a method for producing the negative electrode sheetencompass: a method in which a negative electrode active material ispressure-molded on a negative electrode current collector; and a methodin which (i) a negative electrode active material is formed into a pastewith the use of a suitable organic solvent, (ii) then, a negativeelectrode current collector is coated with the paste, and (iii)subsequently, the paste is dried and then pressured so that the paste isfirmly fixed to the negative electrode current collector. The abovepaste preferably includes the above electrically conductive agent andthe binding agent.

<Nonaqueous Electrolyte>

A nonaqueous electrolyte in a nonaqueous electrolyte secondary batteryin accordance with an embodiment of the present invention is not limitedto any particular one, provided that the nonaqueous electrolyte is onethat is generally used for a nonaqueous electrolyte secondary battery.The nonaqueous electrolyte can be one prepared by, for example,dissolving a lithium salt in an organic solvent. Examples of the lithiumsalt encompass LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, lower aliphatic carboxylic acidlithium salt, and LiAlCl₄. It is possible to use only one kind of theabove lithium salts or two or more kinds of the above lithium salts incombination.

Examples of the organic solvent to be contained in the nonaqueouselectrolyte encompass carbonates, ethers, esters, nitriles, amides,carbamates, and sulfur-containing compounds, and fluorine-containingorganic solvents each obtained by introducing a fluorine group into anyof these organic solvents. It is possible to use only one kind of theabove organic solvents or two or more kinds of the above organicsolvents in combination.

<Method for Producing Nonaqueous Electrolyte Secondary Battery Memberand Method for Producing Nonaqueous Electrolyte Secondary Battery>

A nonaqueous electrolyte secondary battery member in accordance with anembodiment of the present invention can be produced by, for example,disposing the positive electrode, the nonaqueous electrolyte secondarybattery separator in accordance with an embodiment of the presentinvention or the nonaqueous electrolyte secondary battery laminatedseparator in accordance with an embodiment of the present invention, andthe negative electrode in this order.

Further, a nonaqueous electrolyte secondary battery in accordance withan embodiment of the present invention can be produced by, for example,(i) forming a nonaqueous electrolyte secondary battery member by themethod described above, (ii) placing the nonaqueous electrolytesecondary battery member in a container which is to serve as a housingof the nonaqueous electrolyte secondary battery, (iii) filling thecontainer with a nonaqueous electrolyte, and then (iv) hermeticallysealing the container while reducing the pressure inside the container.

EXAMPLES

The following description will discuss embodiments of the presentinvention in greater detail with reference to Examples and ComparativeExamples. Note, however, that the present invention is not limited tothe following Examples and Comparative Examples below.

[Measurement Method]

The following method was used for measurement of physical properties andthe like of a polyolefin porous film produced in each of Examples 1 and2 and Comparative Examples 1 and 2.

[Thickness of Film]

A thickness of the polyolefin porous film was measured with the use of ahigh-precision digital measuring device (VL-50) manufactured by MitutoyoCorporation.

[Air Permeability]

An air permeability of the polyolefin porous film was measured with useof a digital Oken-type air permeability tester EGO1 manufactured byAsahi Seiko Co., Ltd.

[Piercing Strength]

The polyolefin porous film was fixed with a washer of 12 mmΦ. Then, apin (pin diameter: 1 mmΦ and a tip radius of 0.5 R) was pierced throughthe polyolefin porous film at 200 mm/min. A maximum stress (gf) at thetime when the polyolefin porous film was pierced was measured and avalue obtained by this measurement was defined as a piercing strength ofthe polyolefin porous film.

[Shutdown Temperature (SD Temperature)]

A circular measurement sample having a diameter of 19.4 mm was punchedout from the polyolefin porous film, and this sample was set as ameasurement sample. Further, members of 2032 Type Coin Cell(manufactured by Hohsen Corporation) were prepared. The members were atop cover, a bottom cover, a gasket, a kapton ring (having an outerdiameter of 16.4 mm, an inner diameter of 8 mm, and a thickness of 0.05mm), a spacer (circular spacer having a diameter of 15.5 mm and athickness of 0.5 mm), and an aluminum ring (having an outer diameter of16 mm, an inner diameter of 10 mm, and a thickness of 1.6 mm).

Then, the measurement sample and the gasket were placed in this orderfrom the bottom cover and the measurement sample was impregnated with 10μmL of an electrolyte. Thereafter, the kapton ring, the spacer, thealuminum ring, and the top cover were provided above the measurementsample in this order and these members with the measurement sample weresealed with a coin cell caulking device (manufactured by HohsenCorporation), so that a measurement coin cell was prepared. Theelectrolyte used here was a nonaqueous electrolyte (i) obtained bydissolving LiBF₄ in a mixed solvent in which propylene carbonate andNIKKOLBT-12 (manufactured by Nikko Chemicals Co., Ltd.) were mixed at avolume ratio of 91.5:8.5, and (ii) having a temperature of 25° C. and anLiBF₄ concentration of 1.0 mol/L. A temperature inside the measurementcoin cell was continuously measured by use of a Digital Multimeter(7352A, manufactured by ADC CORPORATION) while being increased from aroom temperature to 150° C. at a rate of 15° C./min, and a resistancevalue at 1 kHz in the coin cell was continuously measured by use of anLCR Meter (IM3523, manufactured by HIOKI E.E. CORPORATION).

The coin cell which had a resistance value of not less than 2000Ω at 1kHz during the measurement was considered to have a shutdown function.

In this case, from a graph showing a relationship between a celltemperature and a resistance value, a point of intersection of a tangentto a resistance value of 2000Ω and a straight line, which is anextension line of a base resistance value obtained before the resistancestarted to greatly increase, was assumed to be a shutdown temperature(SD temperature) of the polyolefin porous film.

[Surface Roughness]

<Contact Measurement Method>

As a contact-type surface roughness measurement apparatus, “HANDYSURF(E-35A)” manufactured by Tokyo Seimitsu Co., Ltd. was used. The tip ofthe probe of a measurement head of the contact-type surface roughnessmeasurement apparatus was cone-shaped, with an angle of 60 degrees. Theradius of the tip of the probe was 2 μm.

The surface roughness measurement apparatus was set as follows:measurement force=0.75 mN; measurement speed=0.6 mm/s; evaluationlength=5.0 mm; and cutoff value=0.8 mm. Then, a test piece was preparedsuch that the MD of the polyolefin porous film corresponded to a longside direction, while the TD of the polyolefin porous film correspondedto a short side direction. The surface roughness measurement of the testpiece was carried out twice per direction by a contact measurementmethod.

Respective average values thus obtained in the above directions (the MDand the TD) were defined as a surface roughness Ra_(contact, MD) in theMD obtained by a contact measurement and a surface roughnessRa_(contact, TD) in the TD obtained by a contact measurement.

<Non-Contact Measurement Method>

As a non-contact-type roughness measurement apparatus, “VertScan[registered trademark] 2.0 R5500GML” manufactured by Ryoka Systems Inc.was used. The measurement conditions are as follows.

Objective lens: 5× (Michelson type)

Intermediate lens: 1×

Wavelength filter: 530 nm

CCD camera: ⅓ inch

Measurement mode: Wave

Cutoff: None

The surface roughness of the polyolefin porous film was obtained by anon-contact measurement. The one-dimensional surface roughness Ra for alength of 500 μm was obtained for each of the MD and the TD fromtwo-dimensional data obtained at a measurement point. The aboveoperation was repeated at arbitrarily selected 5 points of thepolyolefin porous film. Respective average values in the abovedirections were calculated from values thus obtained and defined as asurface roughness Ra_(non-contact, MD) in the MD obtained by anon-contact measurement and a surface roughness Ra_(non-contact, TD) inthe TD obtained by a non-contact measurement, respectively.

<Calculating Product of Ra Differences>

A product of Ra differences was calculated by using theRa_(contact, MD), Ra_(non-contact, MD), Ra_(contact, TD), andRa_(non-contact, TD), which had been calculated above.

Example 1

First, 18% by weight of ultra-high molecular weight polyethylene powder(HI-ZEX MILLION 145M, manufactured by Mitsui Chemicals, Inc.) and 2% byweight of hydrogenated petroleum resin (softening point: 90° C.) wereprepared. These powders were pulverized and mixed by a blender, so thata mixed powder was obtained. Here, pulverization was carried out untilparticles of the powders had the same particle diameter.

The mixed powder was then fed to a twin screw kneading extruder by aquantitative feeder, and melt-kneaded in the twin screw kneadingextruder. Subsequently, a resultant melt-kneaded material was extrudedthrough a T-die via a gear pump, so that a sheet-shaped polyolefin resincomposition was obtained. Meanwhile, when the mixed powder wasmelt-kneaded, 80% by weight of liquid paraffin was added under pressureinto the twin screw kneading extruder via a pump, and melt-kneadedtogether with the mixed powder. At this time, an average temperature ofsegment barrels was set at 173.1° C. The segment barrels were segmentbarrels positioned immediately before a section where the liquidparaffin was put in, at the section where the liquid paraffin was putin, and immediately after the section where the liquid paraffin was putin. The polyolefin resin composition was cooled by a cooling roller at40° C., so that a roll of the sheet-shaped polyolefin resin compositionwas obtained. In this case, the circumferential velocity of the coolingroller was set at 1.3 m/min.

Next, the sheet-shaped polyolefin resin composition was stretched at astretch ratio of 6.4 times in the MD at 117° C. and then stretched at astretch ratio of 6.0 times in the TD at 115° C. The polyolefin resincomposition thus stretched was cleaned with use of a cleaning liquid(heptane). Thereafter, the polyolefin resin composition having beencleaned was dried at room temperature, and then placed in an oven at129° C. for heat fixing for 5 minutes, so that a polyolefin porous filmwas obtained. The polyolefin porous film thus produced had a thicknessof 15.7 μm and an air permeability of 115 sec/100 mL.

Example 2

First, 18% by weight of ultra-high molecular weight polyethylene powder(HI-ZEX MILLION 145M, manufactured by Mitsui Chemicals, Inc.) and 2% byweight of hydrogenated petroleum resin (softening point: 125° C.) wereprepared. These powders were pulverized and mixed by a blender, so thata mixed powder was obtained. Here, pulverization was carried out untilparticles of the powders had the same particle diameter.

The mixed powder was then fed to a twin screw kneading extruder by aquantitative feeder, and melt-kneaded in the twin screw kneadingextruder. Subsequently, a resultant melt-kneaded material was extrudedthrough a T-die via a gear pump, so that a sheet-shaped polyolefin resincomposition was obtained. Meanwhile, when the mixed powder wasmelt-kneaded, 80% by weight of liquid paraffin was added under pressureinto the twin screw kneading extruder via a pump, and melt-kneadedtogether with the mixed powder. At this time, an average temperature ofsegment barrels was set at 179.6° C. The segment barrels were segmentbarrels positioned immediately before a section where the liquidparaffin was put in, at the section where the liquid paraffin was putin, and immediately after the section where the liquid paraffin was putin. The polyolefin resin composition was cooled by a cooling roller at40° C., so that a roll of the sheet-shaped polyolefin resin compositionwas obtained. In this case, the circumferential velocity of the coolingroller was set at 1.3 m/min.

Next, the sheet-shaped polyolefin resin composition was stretched at astretch ratio of 6.4 times in the MD at 117° C. and then stretched at astretch ratio of 6.0 times in the TD at 115° C. The polyolefin resincomposition thus stretched was cleaned with use of a cleaning liquid(heptane). Thereafter, the polyolefin resin composition having beencleaned was dried at room temperature, and then placed in an oven at129° C. for heat fixing for 5 minutes, so that a polyolefin porous filmwas obtained. The polyolefin porous film thus produced had a thicknessof 15.7 μm and an air permeability of 86 sec/100 mL.

Comparative Example 1

First, 20% by weight of ultra-high molecular weight polyethylene powder(HI-ZEX MILLION 145M, manufactured by Mitsui Chemicals, Inc.) wasprepared. The powder was then fed to a twin screw kneading extruder by aquantitative feeder, and melt-kneaded in the twin screw kneadingextruder. Subsequently, a resultant melt-kneaded material was extrudedthrough a T-die via a gear pump, so that a sheet-shaped polyolefin resincomposition was obtained. Meanwhile, when the mixed powder wasmelt-kneaded, 80% by weight of liquid paraffin was added under pressureinto the twin screw kneading extruder via a pump, and melt-kneadedtogether with the mixed powder. At this time, an average temperature ofsegment barrels was set at 170.4° C. The segment barrels were segmentbarrels positioned immediately before a section where the liquidparaffin was put in, at the section where the liquid paraffin was putin, and immediately after the section where the liquid paraffin was putin. The polyolefin resin composition was cooled by a cooling roller at40° C., so that a roll of the sheet-shaped polyolefin resin compositionwas obtained. In this case, the circumferential velocity of the coolingroller was set at 1.3 m/min.

Next, the sheet-shaped polyolefin resin composition was stretched at astretch ratio of 6.4 times in the MD at 117° C. and then stretched at astretch ratio of 6.0 times in the TD at 115° C. The polyolefin resincomposition thus stretched was cleaned with use of a cleaning liquid(heptane). Thereafter, the polyolefin resin composition having beencleaned was dried at room temperature, and then placed in an oven at127° C. for heat fixing for 5 minutes, so that a polyolefin porous filmwas obtained. The polyolefin porous film thus produced had a thicknessof 18.8 μm and an air permeability of 157 sec/100 mL.

Comparative Example 2

First, 72% by weight of ultra-high molecular weight polyethylene powder(GUR2024, manufactured by Ticona Corporation) and 28% by weight ofpolyethylene wax (FNP-0115, manufactured by Nippon Seiro Co., Ltd.)having a weight-average molecular weight of 1,000 were prepared, i.e.,100 parts by weight in total of the ultra-high molecular weightpolyethylene and the polyethylene wax were prepared. Then, 0.4 parts byweight of an antioxidant (Irg1010, manufactured by Ciba SpecialtyChemicals Corporation), 0.1 parts by weight of an antioxidant (P168,manufactured by Ciba Specialty Chemicals Corporation), and 1.3 parts byweight of sodium stearate were added to 100 parts by weight by weight ofthe ultra-high molecular weight polyethylene and the polyethylene wax.Then, calcium carbonate (manufactured by Maruo Calcium Co., Ltd.) havingan average particle diameter of 0.1 μm was further added so that thecalcium carbonate accounted for 38% by volume of the total volume. Then,the above materials in powder form were mixed by a Henschel mixer, andwere then melt-kneaded by a twin screw kneading extruder, and thus apolyolefin resin composition was obtained. At this time, a barreltemperature in the twin screw kneading extruder was set at 200° C. Then,the polyolefin resin composition was rolled with use of a pair ofrollers each having a surface temperature of 150° C., so that a sheetwas produced. The sheet thus produced was immersed in a hydrochloricacid aqueous solution (4 mol/L of hydrochloric acid, 0.5% by weight of anonionic surfactant), so that the calcium carbonate was removed. Then,the sheet was stretched at a stretch ratio of 6.2 times at a temperatureof 105° C., so that a polyolefin porous film was obtained. Thepolyolefin porous film thus produced had a thickness of 15.5 μm and anair permeability of 107 sec/100 mL.

[Results]

The above methods were used for measurement of a “product of Radifferences”, a “piercing strength”, and an “SD temperature” of thepolyolefin porous film (nonaqueous electrolyte secondary batteryseparator) produced in each of Examples 1 and 2 and Comparative Examples1 and 2. Table 1 below shows measurement results.

TABLE 1 Product of Ra Piercing SD Differences Strength Temperature [μm²][Gf] [° C.] Example 1 0.0040 467 138 Example 2 0.0191 435 138Comparative 0.0001 610 141 Example 1 Comparative 0.0297 362 133 Example2

[Conclusion]

The following is clear from Table 1. In the case of the nonaqueouselectrolyte secondary battery separator which was produced inComparative Example 1 and had a product of Ra differences of less than0.0020, though the piercing strength was high, the level of safety wasinsufficient since the SD temperature was high. Accordingly, thenonaqueous electrolyte secondary battery separator produced inComparative Example 1 is considered to have an excessively largeproportion of a surface layer portion 1 which is mainly made of acrystalline portion, as illustrated in FIG. 3.

Meanwhile, in the case of the nonaqueous electrolyte secondary batteryseparator which was produced in Comparative Example 2 and had a productof Ra differences of larger than 0.0280, though the SD temperature waslower, the strength was insufficient since the piercing strength was toolow. Accordingly, the nonaqueous electrolyte secondary battery separatorproduced in Comparative Example 2 is considered to have an excessivelylarge proportion of an interior portion 2 which is mainly made of anon-crystalline portion, as illustrated in FIG. 4.

In contrast, in the case of the nonaqueous electrolyte secondary batteryseparators each of which was produced in Example 1 or 2 and had aproduct of Ra differences of not less than 0.0020 and not more than0.0280, the piercing strength was sufficiently high and since the SDtemperature was sufficiently low, the level of safety was alsoexcellent. Accordingly, the nonaqueous electrolyte secondary batteryseparator produced in each of Examples 1 and 2 is considered to have asurface layer portion 1 that is mainly made of a crystalline portion andan interior portion 2 that is mainly made of a non-crystalline portionin a well-balanced manner, as illustrated in FIG. 2.

INDUSTRIAL APPLICABILITY

A nonaqueous electrolyte secondary battery separator in accordance withan embodiment of the present invention is excellent in both strength andlevel of safety. Therefore, the nonaqueous electrolyte secondary batteryseparator can be suitably used for production of a nonaqueouselectrolyte secondary battery having an enhanced level of safety.

REFERENCE SIGNS LIST

-   -   1 surface layer portion    -   2 interior portion

1. A nonaqueous electrolyte secondary battery separator comprising apolyolefin porous film, wherein: in regard to a surface of thenonaqueous electrolyte secondary battery separator, a product obtainedby multiplying (a) a difference between a surface roughness in a machinedirection obtained by a contact measurement and a surface roughness inthe machine direction obtained by a non-contact measurement by (b) adifference between a surface roughness in a transverse directionobtained by a contact measurement and a surface roughness in thetransverse direction obtained by a non-contact measurement is not lessthan 0.0020 and not more than 0.0280.
 2. The nonaqueous electrolytesecondary battery separator as set forth in claim 1, having a filmthickness of not more than 19.5 μm.
 3. A nonaqueous electrolytesecondary battery laminated separator comprising: a nonaqueouselectrolyte secondary battery separator as set forth in claim 1; and aninsulating porous layer.
 4. A nonaqueous electrolyte secondary batterymember comprising: a positive electrode; a nonaqueous electrolytesecondary battery separator as set forth in claim 1; and a negativeelectrode, the positive electrode, the nonaqueous electrolyte secondarybattery separator, and the negative electrode being disposed in thisorder.
 5. A nonaqueous electrolyte secondary battery comprising: anonaqueous electrolyte secondary battery separator as set forth inclaim
 1. 6. A nonaqueous electrolyte secondary battery membercomprising: a positive electrode; a nonaqueous electrolyte secondarybattery laminated separator as set forth in claim 3; and a negativeelectrode, the positive electrode, the nonaqueous electrolyte secondarybattery laminated separator, and the negative electrode being disposedin this order.
 7. A nonaqueous electrolyte secondary battery comprising:a nonaqueous electrolyte secondary battery laminated separator as setforth in claim 3.